In recent years, for the purpose of mounting a lithium ion secondary battery on electric/hybrid automobiles that require large capacity, further increase in capacity and high performance of the battery are requested. As a method for solving the request, there is proposed a method of increasing the charging and discharging capacity by using silicon or tin that stores a large amount of lithium per unit volume or an alloy containing them as a negative electrode active material or by employing the same as a mixture with carbon such as conventional graphite.
However, when an active material having a large charging and discharging capacity, such as silicon or tin or an alloy containing them is used, the active material causes very large volume change resulting from charging and discharging. For the reason, in the case where a rubber-based resin such as polyvinylidene fluoride or SBR, which has been widely used in a conventional electrode that uses carbon such as graphite as an active material, is used as a binder, the resin cannot follow the very large volume change of the active material and thus exfoliation occurs at the interface between the collector and the active material layer. Therefore, there is a problem that a current-collecting structure in the electrode is destroyed, electron conductivity of the electrode decreases, and the cycle properties of the battery easily decrease.
Accordingly, there is requested a binder for a lithium ion secondary battery electrode using a binding resin component capable of following the very large volume change of the active material.
On the other hand, there is known a technology of using a polyvinyl acetal-based resin as a binding resin component in a binder for a lithium ion secondary battery electrode (for example, see Patent Document 1). Such a technology provides a composition that is excellent in adhesive properties between the electrode active material and the collector and is capable of manufacturing a lithium ion secondary battery having a high capacity by using a polyvinyl acetal-based resin in which the amount of the hydroxyl group falls within a specific range. In the document, it is described that the electrode sheet test piece obtained using the technology show a low elution rate in the case where it is allowed to stand in an electrolytic solution at room temperature for 1 hour.
However, even when the above technology is applied, since the binder in the electrode elutes into an electrolytic solution (carbonate ester-based solvent) though the amount is only little, there is a problem in the case of using the battery stably over a long period of time. Moreover, since the degree of swelling of the binder with respect to the electrolytic solution is high, there is a problem of a decrease in discharging capacity retention upon a charging and discharging cycle (see Non-Patent Document 1).
Furthermore, in the electrode using the polyvinyl acetal-based resin as a binder, the binder swollen with the electrolytic solution can stretch following the expansion of the electrode active material but there is a problem that the binder cannot follow the contraction of the electrode active material at the time of discharging.
Therefore, strain is generated and increased between the binder and the electrode active material upon repeated charging and discharging and, as a result, there is a concern that the electrode is exfoliated from the collector owing to breakage of the binder and exfoliation of the active material from the binding resin component, so that there is a problem that a lithium ion secondary battery having a good performance cannot be obtained.
Accordingly, there is requested a resin, as a binder in the electrode, that has a low degree of elution and a low degree of swelling with respect to a carbonate-based electrolytic solution, contracts to the state before stress application when stress is removed after the stress is applied from the electrode active material and the resin is stretched, and can continue to follow the electrode active material even when the expansion and the contraction are repeated.
Moreover, there is known a technology of covering the electrode active material of a lithium ion secondary battery with a crosslinked product of a polyvinyl acetal-based resin (for example, see Patent Document 2). In such a technology, the active material is covered with the crosslinked product of the polyvinyl acetal-based resin for improving the adhesive force between the active material and the binder (binding agent) and suppressing the collapse of the active material, and it is inevitable to use a binder for the active material of the lithium ion secondary battery electrode in combination. However, in the technology, the crosslinked product of the polyvinyl acetal-based resin is only used as a covering agent of the active material and the binder is a conventionally used resin such as polytetrafluoroethylene, a rubber-based resin composed of ethylene-propylene-diene rubber or a styrene-butadiene copolymer, or a polyester amide, so that there are the same problems as described above.